There has been a continuing effort to develop radar systems which are suitable for high-resolution applications, such as ground-mapping and air reconnaissance. Initially, this finer resolution was achieved by the application of pulse-compression techniques to conventional radar systems which were designed to achieve range resolution by the radiation of a short pulse, and angular, or azimuth, resolution by the radiation of a narrow beam. The pulse-compression techniques provided significant improvement in the range resolution of the conventional radar systems, but fine angular resolution by the radiation of a narrow beam still required a large-diameter antenna which was impractical to transport with any significant degree of mobility. Subsequent to the development of pulse-compression techniques, synthetic aperture radar (SAR) techniques were developed for improving the angular resolution of a radar system to a value significantly finer than that directly achievable with a radiated beam width from a conventional antenna of comparable diameter.
In prior techniques, an equivalent to a large-diameter antenna was established which was comprised of a physically long array of antennas, each having a relatively small diameter. In the case of a long antenna array, a number of radiating elements were positioned at sampling points along a straight line and transmission signals were simultaneously fed to each element of the array. The elements were interconnected such that simultaneously received signals were vectorially added to exploit the interference between the signals received by the various elements to provide an effective radiation pattern which was equivalent to the radiation pattern of a single element multiplied by an array factor. That is, the product of a single element radiation pattern and the array factor resulted in an effective antenna pattern having significantly sharper antenna pattern lobes than the antenna pattern of the single element.
SAR systems are based upon the synthesis of an effectively long antenna array by signal processing means rather than by the use of a physically long antenna array. With a SAR, it is possible to generate a synthetic antenna many times longer than any physically large antenna that could be conveniently transported. As a result, for an antenna of given physical dimensions, the SAR will have an effective antenna beam width that is many times narrower than the beam width which is attainable with a conventional radar. In most SAR applications, a single radiating element is translated along a trajectory, to take up sequential sampling positions. At each of these sampling points, a signal is transmitted and the amplitude and the phase of the radar signals received in response to that transmission are stored. After the radiating element has traversed a distance substantially equivalent to the length of the synthetic array, the signals in storage are somewhat similar to the signals that would have been received by the elements of an actual linear array antenna.
A SAR can obtain a resolution similar to a conventional linear array of equivalent length as a consequence of the coherent transmission from the sampling points of the SAR. The stored SAR signals are subjected to an operation which corresponds to that used in forming the effective antenna pattern of a physical linear array. That is, the signals are added vectorially, so that the resulting output of the SAR is substantially the same as could be achieved with the use of a physically long, linear antenna array.
In generating the synthetic antenna, the signal processing equipment of a SAR operates on a basic assumption that the radar platform travels along a straight line trajectory at a constant speed. In practice, an aircraft carrying the radar antenna is subject to deviations from such non-accelerated flight. It is therefore necessary to provide compensation for these perturbations to straight-line motion. This motion compensation must be capable of detecting the deviation of the radar platform path from a true linear path.
Briefly, and referring now to FIG. 1 in the drawings, a SAR system carried by an aircraft 10 maps a target region 12 by transmitting and receiving radar signals at various sampling points S1, . . . , SN, along the flight path 14 of the aircraft. In this regard, the SAR system may be positioned in the nose portion 15 of the aircraft. As the SAR system operates, detection, imaging and identification of moving ground targets can be a critical piece of intelligence. Typically, such detection, imaging and identification is performed with a spatial diversity radar having a multiple phase center antenna, such as a displaced phase center antenna (DPCA) or space-time adaptive processing (STAP) system.
As will be appreciated, the primary goals of such conventional SAR systems are typically to locate a target on the ground, track its motion and form a corresponding SAR image of the target. Unfortunately, location, tracking and imaging can be difficult to accomplish with a single phase center SAR antenna. Several surveillance and tracking radars have been developed for moving target detection and tracking, such as the JSTARS (Joint Surveillance and Target Attack Radar System), which can also form an inverse SAR image of a moving target. Whereas such radar systems have a multiple phase center antenna that allows the signal processor to detect and track a moving target on the ground, these systems are expensive to build. Also, such systems typically require accurate maintenance of the phase calibration among the phase centers.